1. Field of the Invention
The present invention relates to an organic light emitting device and a method for manufacturing the same. More particularly, the invention relates to an organic light emitting device which can prevent image spreading and deterioration in color purity while having enhanced light coupling efficiency, and a method for manufacturing the same.
2. Description of the Related Technology
In general, a light emitting device is a self-emissive device, and has various advantages including a wide viewing angle, excellent contrast and a fast response speed.
The light emitting device (LED) is classified into an inorganic LED and an organic LED according to materials that form an emission layer. Generally, the organic LED (OLED) exhibits better characteristics than the inorganic LED in that it has excellent brightness, a low drive voltage and a fast response speed and enables multiple color display.
An OLED is generally constructed such that an anode is formed on a substrate, and a light emitting portion including at least an emission layer and a cathode are sequentially formed on the anode. The light emitting portion may further include a hole transport layer and an electron transport layer in addition to the emission layer. The hole transport layer, the emission layer and the electron transport layer are organic thin films made of organic compounds.
Light efficiency of an OLED typically includes internal luminous efficiency and external luminous efficiency. The internal luminous efficiency depends upon photoelectric conversion efficiency of organic compounds forming organic layers, including a hole transport layer, an emission layer, an electron transport layer, and so on. The external luminous efficiency (to be referred to as “light coupling efficiency” hereinafter) depends upon the refractive index of each layer.
Compared to the light coupling efficiency of a cathode ray tube (CRT), a plasma display panel (PDP), a field emitter display (FED) or other display devices, the light coupling efficiency of an organic LED is relatively low. This is because while passing through various layers of the device, the light generated from a light emitting portion may become extinct within the device without being transmitted to the air due to total internal reflection. In a rear-emission type OLED, for example, light generally travels in three modes: i) an anode/light emitting portion mode in which light generated at a light emitting portion is totally reflected at an anode/substrate interface to then be guided to the anode or the light emitting portion; ii) a substrate mode in which the generated light is totally reflected at a substrate/air interface to then be guided to the substrate; and iii) an external mode in which the generated light is transmitted to the outside. In an OLED using a 200 nm thick ITO layer, it is known that about 45.2% of light generated at a light emitting portion is associated with the anode/light emitting portion mode, about 31.1% is associated with the substrate mode, and about 23.5% is associated with the external mode. In other words, the light coupling efficiency of a typical OLED is very low, that is, approximately 23%.
One among various attempts to enhance light coupling efficiency of an OLED is associated with the use of diffractive gratings. For example, Korean Patent Publication No. 10-0379396 discloses an OLED panel comprising a substrate having grooves (diffractive gratings) with a predetermined depth, the grooves spaced a predetermined interval apart from each other, an anti-waveguide layer formed in an area where the grooves are formed, a first electrode formed on the entire surface of the substrate, barriers formed on the anti-wave guiding layer, and an organic light emitting layer and a second electrode sequentially formed on the barriers and the first electrode.
Korean Patent Publication No. 2003-0026450 discloses a photonic crystal organic LED comprising a transparent substrate having irregularities formed thereon, a transparent electrode layer formed on the organic substrate, a hole transport organic light emitting layer formed on the transparent electrode layer, an electron transport organic light emitting layer formed on the hole transport organic layer, and a cathode layer formed on the electron transport organic layer.
Japanese Patent No. 2991183 discloses an LED having a stacked structure in which a transparent substrate, a diffraction grating, a transparent electrode, an organic layer, and a reflective electrode are sequentially stacked in that order.
Such a diffraction grating diffracts some of the light guided in the anode/light emitting portion mode to make the same incident into the anode/substrate interface at an angle smaller than a critical angle of total internal reflection to then be externally transmitted. The light that is not transmitted to the outside is guided to and travels along the anode and the light emitting portion. However, the guided light cannot travel to an adjacent pixel due to a pixel define layer (PDL) disposed in the anode/light emitting portion. Even when the guided light can travel to an adjacent pixel, the traveling light is mostly absorbed into the anode and the light emitting portion and then become extinct, thereby exerting substantially no effect on the adjacent pixel.
However, the light that is totally reflected at the substrate/air interface and guided in the substrate mode travels to an adjacent pixel where the guided light is subsequently transmitted to the outside. This is because a PDL for blocking traveling of light is not formed in the substrate, unlike in the anode and the light emitting portion, and the guided light is not removed from the substrate due to a very low light absorption efficiency of the substrate. The light having reached the adjacent pixel is transmitted to the outside by means of a diffraction grating provided at the adjacent pixel, resulting in image spreading and deterioration in color purity and ultimately lowering picture quality of an organic light emitting device.